Cold-rolled ultra low carbon steel sheet with improved press-forming properties

Abstract

The present invention discloses a cold-rolled ultra low carbon steel sheet with improved press-forming properties, which comprises 0.002-0.015% of C, less than 0.006% of N, less than 0.01% of O, less than 0.02% of S, 0.1-0.35% of Mn, 0.01-0.06% of acid-soluble Al, 0.02-0.15% of Ti, and 0.05-0.4% of Cr, and has a value less than 0.8 for the atomic ratio (Ti*/(C+N)) of the total titanium content except titanium of oxide form (Ti*) to the sum of carbon and nitrogen contents, with the balance Fe and incidental impurities. This steel has sufficient values of yield and tensile strengths; is excellent in various properties such as elongation, work hardening index, r value, aging index and Erichsen value; and shows good press-forming properties without any special overaging treatment.

What is claimed as new and desired to be secured by Letters Patent of the United States is: 
     
       1. A cold-rolled ultra low carbon steel sheet with improved press-forming properties, characterized in that said sheet contains 0.002-0.015% of C., less than 0.006% of N, less than 0.01% of O, less than 0.02% of S, 0.1-0.35% of Mn, 0.01-0.06% of acid-soluble Al, 0.02-0.15% of Ti, 0.05-0.4% of Cr, and one or both of 0.001-0.1% of a rare earth element and 0.001-0.01% of B, and has a value of less than 0.8 for the atomic ratio (Ti*/(C+N)) of the total titanium content except titanium of oxide form (Ti*) to the sum of carbon and nitrogen contents, with the balance Fe and incidental impurities, wherein said steel sheet has an aging index of no more than 4.5 kg/mm 2 . 
     
     
       2. The steel sheet of claim 1, additionally containing Si, wherein the Si-content is less than 0.1%.